Smallest lifeforms on Earth could shake up the tree of life as we know it

Scientists identify more than 35 new groups of bacteria that don't belong on the traditional tree.

Only a small fraction of bacterial phyla can be grown in culture. (Photo: Rocky Mountain Laboratories, NIAID, NIH)

When contemplating the tree of life, the picture in your mind is probably populated by animals, plants and maybe some fungi. But actually, these larger, multicellular organisms make up only a small portion of life on Earth. Most life is actually microbial — bacterial or archaebacterial.

Now scientists from the University of California, Berkeley, have identified at least 35 more new groups of bacteria — including the smallest known lifeforms ever found — and they're saying the discovery could forever change how we classify life, according to a news release.

To get a grasp on just how startling this discovery is, consider that the number of newly added bacterial phyla alone is equal to all the known animal phyla on Earth. And these new phyla are unlike anything ever identified before; they represent entirely new branches on the tree of life.

"This is a new view of the tree of life," said lead author Jill Banfield. "These new major features on the tree of life mean that it probably won't be the simple three-domain view we have now."

Traditionally, the tree of life is separated into three primary domains: Archaea, Bacteria and Eukaryota. We, along with all multicellular organisms, belong in Eukaryota, because our cells have a nucleus. Organisms in both Archaea and Bacteria consist of single-celled microbes that have no nucleus, but both groups are still distinct enough from one another to be classified in entirely different domains. The biodiversity of both of these domains is profuse. Scientists estimate there are more than 100 different phyla of bacteria, for instance.

Since only 29 bacterial phyla have representatives that will grow in culture, our understanding of most of the microbial realm is scant, to say the least. The number of new phyla identified by the Berkeley scientists thus inflates our classification of known organisms in an unparalleled fashion. Some of these new bacteria are so unusual that scientists may even need to consider adding them to whole new domains of life separate from the big three.

Though many of the new species of bacteria were known to exist, they had never been properly identified, partly because they can't be grown in culture and also because many of them are so small — the tiniest organisms ever identified. These were essentially stripped-down life forms with only the barebones requirement of genes. But the Berkeley researchers were able to isolate and sequence their DNA for the first time using a technique known as metagenomic analysis.

"We were really surprised to find how diverse these groups are within the bacterial domain, and just how consistently different the organisms within this radiation are from the rest of bacteria," said graduate student Christopher Brown, who was part of the research team. "No one had been able to put all the pieces together before."

In fact, the new groups are so diverse that the researchers estimate there may be more than 250 separate phyla within this radiation.

"I think what this is telling us is that a large part of bacteria and bacterial lifestyles are very different from what we thought before. There is a lot of biology that we haven't been able to understand from our current methods," said Brown.